Is there a ban by mac address somewhere?

Hi, I have a total strange problem:
in my network are several computers. And two of them - normally used by one person - have problems accessing the server.
One machine runs Xubuntu and the other is a tablet running Android.
nslookup on the machines work. And sometimes I can also browse websites beeing on the ispconfig 3 server.
But ... very often, there is the problem, that I can't open any domain hosted on this ispconfig server.
At the same time the machine in question can open other websites without any problems.
Also a telnet domain.com 80 does not work.
I tried accessing the server (domains, telnet) from my MacBook at the same time the other users has the problems. Here, I find no problems. The same is for reaching the domains using a smartfone.
ALL machines are within the same network and use the same public IP!!! And all machines go through the same router (a FritzBox).
Anyone an idea?